Know this: if parts are touching (SURFACES are coincident) you can automatically build connections with little effort. If the contacting surfaces are not coincedent or you wan to connect edges or even vertices to a surface you can use the manual contact definitions. A rigid connector is primarily designed to work over larger distances not to define a "contact". That is a simplified set of the "rules" but you can go a long way on that if you are just using solid element models.